Known are piezo-electric transducers of the Tonpilz type comprising a stack of piezo-electric elements alternating with electrodes, which stack is placed between, on the one hand, a front receiver plate whose external front face is the emittive or receptive face, and on the other hand, a countermass. These transducers are generally of a total length which is the order of one-half of the fundamental wavelength in the material constituting the transducer.
As a result, transducers of the Tonpilz type adapted for emitting or receiving acoustical waves of low frequency are of relatively large size. For example, a transducer for emitting acoustical waves having a frequency of 1300 Hz has a length of the order of one meter and antennas composed of a number of transducers of this type are very heavy and very cumbersome. On the other hand, the stack of piezo-electric elements and the countermass are generally placed in a sealed casing filled with air in order to avoid the emission of acoustical waves towards the rear and for obtaining transducers having a good intrinsic directivity. When the size of the stack and the casing are substantial, it becomes very difficult to obtain casings which resist the hydrostatic pressures due to immersion at great depths.
Attempts have been made to reduce the length of transducers of Tonpilz type for low frequency by utilizing countermasses of a very dense metal having a low mechanical stiffness, for example, tungsten. However, this solution does not permit reducing the length to a large degree and increases the weight.